The invention relates to a method for determining a deviation of a broadband measurement signal, especially a radar measurement signal, from a reference signal, and a corresponding measuring device.
The use of recurring continuous-waveform signals, referred to in the following as measurement signals, is established in radar technology. The term continuous-waveform signal (CW), covers electromagnetic waves of constant amplitude and frequency. In modern automotive radar technology, frequency-modulated continuous waveforms are used, for example, to realize distance-control systems, distance-warning systems or parking aids in automobiles.
With these radar systems, a continuous waveform is transmitted and a possible reflection of the signal from objects in the environment of the radar system is received. Such signals are linear-frequency-modulated, wherein the linearity of the frequency modulation provides different gradients. From the comparison between the received measurement signal and a transmitted reference signal, inferences can be drawn regarding the distance and relative velocity between the transmitter and an object. In particular, the time delay and the offset (Doppler) between the transmitted reference signal and the received measurement signal are evaluated in this context.
The publication “Radar Waveform for Automotive Radar Systems and Applications” by the authors Rohling and Möller, published at the IEEE Radar Conference in May 2008, describes an automotive radar system in which velocity-resolving signals and distance-resolving signals are investigated. In this context, linear-frequency-modulated continuous waveforms are used. These signals are at least partially linear.
The greater the bandwidth of the measurement signal, the greater the depth resolution of the radar system. For radar systems, which operate in a frequency band from 77 GHz to 81 GHz, the bandwidth of such a measurement signal is typically 2 GHz.
In order to evaluate the accuracy of a radar system, a deviation of the linearity of the measurement signal from the linearity of a reference signal is determined. Alternatively, the phase response of such a signal is investigated. A deviation of the measurement signal from an ideal characteristic, referred to below as the reference signal, has a negative effect on the accuracy of the radar system and accordingly reduces the performance of the system. The deviations of the measurement signal from the reference signal caused by the linearity error are within the range of a few KHz. Such a broadband measurement signal cannot be evaluated with regard to this error expectation using currently available signal analyzers.
What is needed, therefore, is a method and a measuring device for determining a deviation of a broadband measurement signal from a reference signal in which the analysis bandwidth of a measuring device is substantially smaller than the bandwidth of the measurement signal.